In small bowl, with wire whisk, lightly beat egg yolks. In thin steady stream, whisk about 1/2 cup hot milk mixture into yolks. Gradually whisk yolk mixture back into milk mixture, stirring rapidly to prevent curdling. Cook over low heat until mixture is very thick and coats the back of a spoon well or temperature on instant-read thermometer reaches 160 degrees F, stirring constantly. Remove saucepan from heat.